The present invention relates to wavelength division multiplexing (WDM) multichannel communication systems propagating WDM optical signal encoded with a large amount of data via an optical fiber. More particularly, the present invention relates to WDM communication systems utilizing a plurality of WDM optical sources having stabilized wavelengths and light intensity characteristics.
The performance of multichannel WDM optical communication systems requires the integrity of signals produced by WDM optical sources. For contemporary WDM optical communication systems the wavelength and power of each WDM optical source have to remain stable for at least 10 years. Since aging of components and environmental variation cause the wavelength and power drifts, an active stabilization of wavelength and power of each WDM source has to be provided.
A conventional method of control and stabilization of the optical power of each channel of WDM optical transmission system is disclosed, for example by Onaka et al (the U.S. Pat. No. 5,894,362). FIG. 1 of accompanying drawings is a diagram illustrating Onaka et al teaching. WDM multichannel communication system 100 comprises a plurality of diode lasers 101 connected to a corresponding plurality of data modulators 102. A WDM multiplexer 103 combines optical outputs of the data modulators 102. A small portion of the multiplexed optical power is tapped by coupler 104 and launched through a transmission line 105 to a wavelength selective device 106. The wavelength selective device may be a spectrum analyzer having a resolution higher than the WDM channels spacing. The spectrum of the sample signal is then analyzed by a microprocessor 107. The output signals of the microprocessor is used to control output power of the diode lasers 101.
Theoretically this method could also be used for the wavelength stabilization of the high performance WDM communication systems. However it is very difficult to implement it in practice since to stabilize the center wavelength of each laser to the accuracy of the order of 0.01 nm, the spectrum analyzer must have very high spectral resolution.
A conventional method of wavelength stabilization is illustrated by diagram shown in FIG. 2. Each optical source (usually a semiconductor diode laser) 201 of a WDM communication system generates an optical signal. A small portion of the output from each optical source 201 is diverted by an external tap coupler 202 and directed to a wavelength reference device 203, commonly referred to as xe2x80x9cwavelength lockerxe2x80x9d. The error signal generated by the wavelength locker is then fed back to a laser temperature control unit 204 (as described in U.S. Pat. No. 5,798,859; No. 5,825,792; and No. 6,005,995.) The laser temperature control unit 204 shifts the laser wavelength to the reference wavelength. Some commercially available diode lasers have built-in wavelength lockers (U.S. Pat. No. 5,825,792). More frequently, however, external wavelength lockers are used for wavelength stabilization (U.S. Pat. Nos. 5,798,859 and 6,005,995).
Customarily each WDM source has its own wavelength locker. As the number of WDM channels increases, the number of required wavelength lockers increases proportionally. The cost of an external wavelength locker is comparable or even higher than the cost of a WDM source. In addition, the spectrum analyzer used for channel power monitoring is extremely expensive. As a result, the stabilization devices represent a significant portion of the total cost of the multichannel WDM system.
The present invention provides a cost effective, integrated solution for the wavelength and power stabilization of all channels in a WDM communication system, where a low frequency, small depth modulation is superimposed on the light intensity of each individual WDM channel before the corresponding channel is multiplexed by the WDM muliplexer. After WDM channels are multiplexed, modulations having a specific frequency for each channel, serve as identifiers for the corresponding channels. By applying a Fourier transform on total light intensity of the combined optical signal, the characteristic frequency components, and related information, of each channel is extracted.
According to the present invention a WDM communication system for propagating a plurality of optical signals produced by a corresponding plurality of WDM optical sources via an optical fiber comprises a transmission system for generating and transmitting optical signals. The optical signals are modulated with distinguishing low frequency electrical signals having small modulation depth and used as identifiers for each WDM optical source. The modulated optical signals are mixed by the WDM multiplexer. To detect a small portion (about 1%) of WDM optical signal and obtain first and second electrical signals carrying information on light intensity and wavelength of each WDM optical source, a detection system is coupled to the transmission system. A control system is inserted between the transmission system and the detection system for analyzing the first and second electrical signals by Fourier transform and obtaining information on wavelength and light intensity for each WDM optical source. Wavelength and light intensity are adjusted accordingly.
According to one aspect of the present invention the transmission system comprises a plurality of data modulators that are connected respectively to the plurality of WDM optical sources for modulating the optical signal in each WDM channel by an electrical signal of low frequency and small depth of modulation. A low frequency generator is coupled to each data modulator for generating a distinguishing electrical signal of low frequency and small modulation depth. Each data modulator comprises high frequency input for transmitting the optical signal of the WDM optical source and low frequency input for applying the distinguishing electrical signal. A plurality of variable optical attenuators is coupled to a respective plurality of data modulators for setting a predetermined optical power for each WDM optical source. A WDM multiplexer combines output signals of the variable optical attenuators into a WDM optical signal. About 1% of WDM optical signal is diverted by an external tap coupler. A splitter divides a diverted portion of WDM optical signal into first and second sample signals for directing them into respective first and second transmission paths. A first detector is placed within the first transmission path for detecting and converting this first sample signal into a first electrical signal. A wavelength locker and a second detector are placed within the second transmission path for providing wavelength selectivity of the second sample signal and converting it into a second electrical signal. A microprocessor is coupled to the output of the first and second detectors for analyzing first and second electrical signals by transforming them into Fourier components corresponding respectively to intensity and wavelengths of the WDM optical sources. Feedback connectors are coupled between the microprocessor and WDM optical sources for providing digital feedback on wavelength characteristics of WDM optical sources, and between the microprocessor and variable optical attenuators for providing digital feedback on light intensity of WDM optical sources.
According to another aspect of the present invention, a plurality of LiNbO3 modulators are utilized as data modulators. A bias control unit coupled to each LiNbO3 modulator generates a distinguishing electrical signal of low frequency and small modulation depth. Each LiNbO3 modulator comprises high frequency input for transmitting the optical signal of the WDM optical source and low frequency input for applying the distinguishing electrical signal of low frequency and small modulation depth.
According to yet another aspect of the present invention, directly modulated diode lasers are utilized as both WDM optical sources and data modulators. Each directly modulated diode laser is coupled to a generator of low frequency electrical signal. The generator of low frequency electrical signals generates a distinguishing electrical signal of low frequency and small modulation depth that is applied to the low frequency input of a corresponding directly modulated diode laser. Each distinguishing low frequency electrical signal serves as an identifier for a respective directly modulated diode laser for the process of identification and adjusting wavelength and light intensity for each directly modulated diode laser.
According to the method for stabilization of light intensity and wavelength of WDM optical sources of multichannel WDM communication system, the WDM optical sources are modulated by distinguishing electrical signals of low frequency and small modulation depth for obtaining a unique optical output from each WDM optical source. A predetermined optical power is set for WDM optical sources by attenuating each output of the WDM optical sources by variable optical attenuators. The modulated outputs of the WDM optical sources are combined by a WDM multiplexer for obtaining a WDM optical signal. A sample signal, being a small portion of the WDM optical signal, is diverted and divided into two approximately equal portions that are transmitted via two respective transmission paths. The first portion is detected and converted into a first electrical signal. For the second portion, wavelength selectivity is provided by a wavelength locker, and the second portion is converted into a second electrical signal. The first and second signals are analyzed by a microprocessor The microprocessor is equipped with analog-to-digital converters and is connected to outputs of the first and second detectors for transforming them into Fourier components corresponding respectively to wavelengths and light intensity of the WDM optical sources. The microprocessor then analyzes the relative amplitude of each Fourier component and provides digital feedback to set correct wavelength and light intensity of each WDM source respectively. Identification of light intensity and wavelength of each WDM optical source is provided by utilizing the unique optical output for each WDM optical source. Digital feedback is fed to variable optical attenuators and WDM optical sources for stabilization intensities and wavelengths thereof.